Human fibroblasts "senesce" in culture: after a certain number of divisions all cells from normal tissues eventually lose the ability to replicate further. It has been theorized that cell senescence is the cause of tissue aging and organismal death. Several studies aimed at determining the relation between donor age and cell division potential in culture have yielded conflicting conclusions. The traditional method of explant outgrowth and serial high density passage has permitted individual variability with respect to cell density in tissue samples and colony forming ability of the cells to influence the calculation of division potential. Study of only the fibroblast has weakened the strength of inferences drawn about the mechanism of cell senescence and its relation to the aging process. The replicative lifespan in culture of the human epidermal keratinocyte is largely determined by its tendency to terminally differentiate, and its inherent division capacity is more completely attined when EGF is added to cultures. Accurate determination of keratinocyte division capacity has been made by initiating primary cultures and serially passaging cells at plating densities low enough to permit quantification of the colony-forming cells at each subculture. It is proposed that this regimen be used to measure and compare the in vitro division capacities of both the dermal fibroblast and the epidermal keratinocyte populations of human skin. Optimal medial for each cell type will be used to maximize expression of division potential. The population variance of replicative life-span within three narrow age groups will be calculated for each cell type, as well as the correlation between the lifespans of the two cell types within individuals. A popular theory of cell senescence is that it represents a programmed terminal differentiation process for somatic cells. A terminally differentiated state has not yet been identified for the fibroblast, but keratinocytes must enter a post-replicative state as part of their differentiation program. Hybrids will be formed between fibroblast and keratinocytes to test the hypothesis that different systems operate to cause senescence in these different cell types.